Since cellulose is the major structural constituent of most plant matter, it is natural that those interested in processing or refining such materials refer to them as cellulosics. However this general term connotes a multiplicity of meanings whereby each is qualified by descriptors frequently specific to the interest at hand. The commercial applications of processed plant matter to produce a refined cellulosic material are numerous and involve use in many nonanalogous arts. For example, refined celluloses are extensively used in paper and textile applications. Refined cellulose is also used in adhesives, food ingredients, industrial coatings and various other diverse applications. For each end use, the raw material, processing and final product(s) comprise a technological field essentially unique to itself.
In general, a wide variety of chemical, thermal and mechanical transformations are known in the art to refine, manipulate and modify cellulose for numerous purposes. The following hierarchical characterization has been devised to describe previously known technology relating to structural manipulation of refined cellulosic substances. This characterization serves the additional purpose of providing bases for distinction between the process of the present invention and the prior art.
The molecular level or primary structure of cellulose is the beta 1-4 glucan chain. All celluloses share this level of structure and it is the distinguishing difference between cellulose and other complex polysaccharides. The natural chain length is not known due to unavoidable modification and degradation which occurs during the disassembly to this level, but probably extends into the polymerization regimes of many thousands of glucan units. Transformations at this level of structure involve forming and breaking chemical bonds.
Secondary structure is considered to be submicroscopic strands formed from parallel, aligned assemblages of glucan chains. This level of organization is designated the microfibril. Microfibrils are spontaneously formed from a plurality of nascent glucan chains believed to be synthesized simultaneously by a complex, motile, biosynthetic organelle involved in the assembly of the primary plant cell wall. The microfibril is of sufficient size to be discernable with the electron microscope and depending on the plant species ranges in its major cross-sectional dimension from approximately 50 to 100 Angstroms. As with the beta-glucan chain, of which it is composed, the length is indeterminant. Non-covalent interactions, such as by hydrogen bonding, stabilize secondary structure. Because the interchain attraction is high, structural transformation is probably rare unless preceded by chemical modification of primary structure.
Tertiary structure is related to arrays and associations of microfibrils into sheets and larger stranded structures designated fibrils. The distinguishing features at this level of structure are sufficiently small that resolution is usually possible only via the electron microscope. However, some individual fibril assemblies are of sufficient gross cross-sectional dimension (0.1 to 0.5 microns or 10,000 to 50,000 Angstroms) to be discernible with the light microscope. Structural deformation at this level is largely mechanical and either organized (disassembly/denomination) or random (indiscriminate fracture/cleavage).
Lastly, quaternary structure deals with the construct of tertiary elements which form the primary and secondary cell wall. This level of structure defines the physical dimensions of the individual cell and any gross structural specialization required for physiological function of the differentiated cell. Examples are libriform, tracheid and parenchymal cell structure. Structural manipulation results from indiscriminent comminution and is the most commonly employed mechanical transformation practiced.
Conventional pulping of cellulosic materials is primarily concerned with chemi-thermomechanical processing of schlerenchymous or structural plant tissue to achieve individually dispersed cells. The result is a quaternary structure largely consisting of cellulose derived from the primary and secondary cell walls. Depending on the plant source and extent of processing some heteropolysaccharides such as hemicellulose (xylans, galactomannans, pectins, etc.) may also be present. The important distinction of pulping from other processing of celluloses is that an anatomical destruction of intact plant tissue occurs. This results in dispersed cellforms which represent a minimal degree of quaternary and more basic structural levels of manipulation. Some forms of cellulose, such as cotton, are produced naturally in a dispersed state and do not require pulping as a prerequisite.
Important to the following discussion is the distinction between disassembly and indiscriminate fragmentation processes. In fragmentation the localized energy excursion (by whatever means) is sufficiently high and accumulates sufficiently rapidly that an organized dissipation of internal energy by the acquiring matrix is not effected. Here an intense perturbation is applied and results in an indiscriminate fracture or other major disorganization at translocations within a defined microdomain. In the case of disassembly, on the other hand, the acquired energy excursion is dissipated in a more organized manner usually following a path of lowest activation energy. For cellulose this appears to involve segmentation along parallel fibril oriented assemblies and possibly laminar sheet separation of fibril arrays.
Mechanically beaten celluloses have long been employed in the paper and packaging industry. Chemi-thermomechanically refined wood pulps are typically dispersed in hydrobeaters and then subjected to wet refining in high speed disc mills. This level of structural manipulation as presently practiced is exclusively at the quaternary level. The objective of such processing is to disperse aggregated fiber bundles and increase available surface area for contact during drying to increase dry strength. Substantial size reduction and concomitant impairment of dewatering are undesirable and circumscribe the extent of processing. The measurement of the ease of water drainage from a beaten pulp is termed Canadian Standard Freeness and reflects the ease or rate of interstitial water removal from the paper stock.
Finely ground or fragmented celluloses are well known. These products are produced by mechanical comminution or grinding of dried, refined cellulose. They are employed largely as inert, non-mineral fillers in processed foods and plastics. The manipulation is exclusively at the quaternary level of structure. It is achieved by application of a variety of size reduction technologies, such as ball and bar mills, high speed cutters, disc mills or other techniques described in part in U.S. Pat. No. 5,026,569. The practical limit of dry grinding is restricted in part by the thermal consequences of such processing on cellulose and in part to the economics of equipment wear and material contamination of the product. Micromilled cellulose (MMC) prepared in aqueous or other liquid media as described in U.S. Pat. No. 4,761,203 avoids the thermal decomposition associated with prolonged or intense dry grinding. This technique allows particle size reduction into the colloidal range (about 10 microns). It is believed to operate by indiscriminate micro-fragmentation of quaternary structure, without incurring the fusion/thermal degrading effects characteristic of dry grinding.
Microfibrillated cellulose (MFC), as disclosed by Turbak et al (U.S. Pat. No. 4,374,702), is principally a mechanical manipulation of refined cellulose from wood pulp at the tertiary level of structure. The process employs high pressure, impact discharge onto a solid surface of a cellulosic dispersion in a liquid medium. This results in a combination of direct energy transfer through high, adiabatic shear gradients generated within the impact domain and secondary effects of such shear (or translational momentum exchange) from solvent cavitation to disassemble suspended cellulose particles. Depending on the extent of processing and preconditioning of the raw material the structural manipulation produces fibril ensembles of disassembled quaternary structure. These highly dispersed fibril structures impart unusual properties to the continuous liquid phase in which they are prepared.
Microcrystalline cellulose (MCC), as disclosed in U.S. Pat. No. 3,023,104, exemplifies structural manipulation which can occur at the secondary level of structure. The process involves selective acid hydrolysis of solvent accessible and amorphous regions of secondary structure in refined cellulose to produce relatively crystalline microdomains that are resistant to further hydrolysis. The dimension of the crystallite domains is on the order of ten to thirty microns. If the never dried crystallite is sheared, it disperses into parallel clusters of microfibrils, reflecting periodic cleavage along a fibril assembly. The microfibril crystallites exhibit high surface area and readily reassociate on drying into a hard, non-dispersible mass.
Furthermore, the production of rayon and cellulose ethers such as cellulose gum (carboxymethyl cellulose, CMC) involves manipulation at the primary level of structure. In the case of rayon the modification is transient and reversible whereby the reconstituted beta-glucan chain spontaneously reassembles into semi-crystalline material that can be spun into fibrils. Cellulose ethers represent a deliberate, irreversible modification whereby the individually formed beta-glucan chains are prevented from reassembly due to the chemical derivatization. A limited variation of such derivatization is that of powdered cellulose wherein the degree of substitution is relatively low, to form e.g. forming carboxymethyl or diethyl aminoethyl cellulose, CM cellulose and DEAE cellulose, respectively. The latter materials are useful as ion exchange media.